This present invention relates to an optical filter for use with telescopes; specifically to a diffracting filter composed of a plurality of spaced translucent cells separated from each other by a thin opaque border in which the spacing of the cells is proportional to the Airy disk of a specific optical system for the telescope to aid observation through a turbulent atmosphere.
Turbulent atmospheric phenomena limit the visual range of observations in the 380 nm to 750 nm wavelength (λ) range of visible light. All forms of telescopic astronomical investigation will benefit from such a device; ranging from casual visual observations to in-depth detailed scientific imaging, spectroscopic, photometric, astrometric, and interferometric investigations.
Driven by the thermal energy of the sun, the motion of the earth's atmosphere changes the index of refraction and therefore the optical path length of light passing through the atmosphere and entering the telescope. The light which reaches the telescope is distorted by this turbulent atmosphere even though it started as an almost perfect planar wave front. Atmospheric turbulence changes occur in milliseconds and this led to the development of adaptive optical systems. Typical astronomical telescopic observation suggests that isoplanar wave fronts are only about 10 cm square. At astronomical observatories usually on top of mountain ranges, the isoplanar wave front may be as large as 20 to 30 cm when the “seeing is good.” Over any isoplanar region, the wavefront is relatively smooth and provides little curvature with a difference of about λ/17. A rule of thumb adopted by astronomical observers is that if wave distortions are less than λ/10 the image quality will be good.
Since telescopes can only collect a portion of the incident light to be reformed into an image, there will always be some diffraction in all systems. The light will deviate from straight-line propagation and spread out somewhat in the image plane. The effect of turbulence on an image formed by a telescope depends on the size of the aperture of the telescope. If the image is formed from a very small aperture, most of the wave front will be undistorted and planar. The larger the diameter of light allowed causes the Airy disk to be formed which moves as the wave front is distorted. The image is thus a superposition of shifting Airy disk spots resulting in a shimmering blur. The larger aperture will collect more light but will not proportionately improve the resolution of the optical system.
The critical aperture size at which atmospheric blurring occurs is a measure of the turbulence. This aperture is called the Fried parameter, r0, and corresponds to the size of the region over which the incoming wavefront can be taken to be essentially planar. When this parameter r0 exceeds 30 cm, a very distant star will be perfectly imaged as an Airy disk. As the turbulence increases, the Fried parameter r0 decreases. Furthermore, as the wavelength increases, the Fried parameter r0 increases, since the Fried parameter is proportional to the wavelength λ. The resolution of a telescope in the atmosphere is no better than 1.22λ/r0 and since r0 is rarely above 20 cm, even the largest telescopes have a resolving power little more than that of a 8″ telescope. Adaptive optics reduces the sizes of the isoplanar mirror assemblies and then coordinates them based upon atmospheric turbulence, but is both complicated and expensive and beyond the budget of the amateur astronomical observer.
When a telescope with a circular aperture receives plane waves, rather than being an image “point” the light actually spreads out into a small circular spot called the Airy disk containing about 84% of the energy surrounded by several faint rings. The radius of the Airy disk determines the overlapping of the neighboring images and therefore the resolution of the telescope. A telescope which is as perfect as may be possible is referred to as a diffraction-limited telescope. In a turbulent atmosphere, light traveling through the atmosphere is diffracted and therefore arrives out-of-phase with collimated light entering the atmosphere from the cosmological source. The turbulent atmosphere causes starlight to “twinkle” but makes viewing less clear. By placing a diffraction grating over the light path, the refracted starlight is eliminated and the undiffracted portion of the light is allowed to pass through the Atmospheric Stabilization Filter to the focal plane of the instrument, which may either be the pupil of the observer or the focal plane of a CCD camera.
Active optics requires the precise alignment of the main mirror in a large telescope, in conjunction with the precise alignment of the secondary mirror and any other auxiliary optics used in the optical system. Typically active optics is not a continuous real-time alignment system, but rather is employed either in hourly increments during an observation run or immediately before each observation run. Substantial and expensive equipment support equipment and technician time is required for either type of stabilization system. Maintenance of such systems often interferes with observation time, limiting and therefore decreasing time-critical investigations.
Currently, there is no known device, passive in nature (i.e. having no moving parts, and no electrical apparatus) that attaches to a telescope and suppresses atmospheric turbulence. Most devices dealing with astronomical observations through atmospheric turbulence rely upon adaptive optics, or active optics. Both adaptive optics and active optics involve considerable technical sophistication and are limited to small areas of the sky. Adaptive optics for telescopic use in real time observations comprises a special deformable mirror, wavefront sensor, and circuitry intended to match the distortion of the atmosphere to selective movement of the deformable mirror system. A special laser is used to create an artificial guide-star for tracking purposes. Special computers and software must be accessed to automatically make the adaptive movements to the mirror system.